During each cardiac cycle, the natural heart valves alternatively open to allow blood to flow through them and then close to block blood flow. During systole, the mitral and tricuspid valves close to prevent reverse blood flow from the ventricles to the atria. At the same time, the aortic and pulmonary valves open to allow blood flow into the aorta and pulmonary arteries. Conversely, during diastole, the aortic and pulmonary valves close to prevent reverse blood flow from the aorta and pulmonary arteries into the ventricles, and the mitral and tricuspid valves open to allow blood flow into the ventricles. The cardiac valves open and close passively in response to blood pressure changes operating against the valve leaflet structure. Their valve leaflets close when forward pressure gradient reverses and urges blood flow backward and open when forward pressure gradient urges blood flow forward.
In certain individuals, the performance of a natural heart valve is compromised due to a birth defect or becomes compromised due to various disease processes. Surgical repair or replacement of the natural heart valve is considered when the natural heart valve is impaired to an extent such that normal cardiac function cannot be maintained. The natural heart valve can be replaced by homograft valves obtained from the same species (e.g., human donor heart valves), heterograft valves acquired from different species, and prosthetic mechanical heart valves.
The present invention is directed to improvements in prosthetic mechanical heart valves. Modern implantable mechanical heart valves are typically formed of a relatively rigid, generally annular valve body defining a blood flow orifice and an annular valve seat and one or more occluders that are movable between a closed, seated position in the annular valve seat and an open position at an angle to the valve body axis. These components of mechanical heart valves are made of blood compatible, non-thrombogenic materials, e.g., pyrolytic carbon and titanium. A bio-compatible, fabric sewing ring is typically provided around the exterior of the valve body to provide an attachment site for suturing the valve prosthesis into a prepared valve annulus. The occluder(s) is retained and a prescribed range of motion is defined by a cooperating hinge mechanism or other restraining mechanism. Such prosthetic heart valves function essentially as check valves in which the occluder(s) responds to changes in the relative blood pressure in the forward and reverse directions as described above and move between their open and closed positions.
Two approaches to mechanical heart valve design have been followed over the years. In a first approach, the design of the mechanical heart valve structure has attempted to mimic natural heart valve structures in construction, appearance and function. For example, in U.S. Pat. No. 4,556,996, a valve design is proposed using molded elastomer, triangular flaps that extend inwardly into the annulus of a ring shaped valve body that appears to be intended to mimic tricuspid heart valves. The flaps and ring-shaped body are integrally formed of Delrin or a similar hard plastic and covered with an elastomer. The flaps are intended to bend between open and closed positions by integrally formed hinges at the junctions of the flaps and the ring shaped body.
This approach has also led to a number of proposed designs to mimic the operation of a natural tricuspid valve employing flaps formed of thin plastic membranes attached to the valve body and to struts extending downstream from the valve body leaving the flaps with free flap edges. In operation, the three flaps balloon outward in the open position to define a cylindrical annulus for blood flow. In the closed position, the free flap edges of the three flaps collapse against one another. A variety of such mechanical heart valve prostheses are described in U.S. Pat. Nos. 4,222,126, 4,364,127, 5,500,016 and 5,562,729, incorporated herein by reference.
The flexible valve leaflets of the designs following this first approach have not been successfully clinically implemented in part because the leaflet materials and integral hinge mechanisms cannot be shown to be reliable and immune from fracture or tear over long term use. It is also well known that calcium mineral deposits on the flaps causes calcification of leaflets. The calcified leaflets become rigid and fail to open and close properly. Their durability are greatly reduced and valve failure always occurs at the calcified location. Moreover, the integral hinge structures are in low blood flow regions and blood stagnation in those regions can contribute to the accretion of thrombus formation and also cause the failure of these valves.
In the second approach, less attention is paid to trying to mimic the appearance and function of natural heart valve flaps, and more attention is paid to maximizing reliability of operation and hemodynamic function. Such mechanical heart valve prostheses have employed other occluders and hinge or occluder restraint mechanisms that do not resemble flaps and integral flap hinges. A wide variety of such mechanical heart valve designs have been proposed and/or clinically used in the past. For example, U.S. Pat. No. 3,911,502 describes mechanical heart valves employing a spherical ball in a cage that moves in the cage into and out of engagement with an annular valve body seat in response to the blood flow due to normal pumping action of the heart. The spherical ball was formed of a variety of materials including metals, plastics, and silicone rubber.
Other early heart valve prostheses employed occluders in the form of a circular disc restrained within cage struts or by disk mounted struts for movement between open and closed disk positions in response to blood pressure changes, as shown, for example in U.S. Pat. Nos. 3,722,004 and 3,396,409. In the '004 patent, the disk is formed of a pyrolytic carbon or metal ring coated with silicone rubber except for the periphery 20. The periphery 20 contacts the sides of the struts to restrain movement of the disk between the disk open and disk closed positions. The silicone rubber strikes the ends of the struts to stop movement of the disk in the disk open position, and the silicone rubber coating flexes to reduce noise and shock.
Heart valve prostheses using such spherical ball or circular disk occluders provide poor hemodynamic function since the major surfaces of each such occluder remain perpendicular to the blood flow when the occluder is in the open position and therefore they impede blood flow. These types of valve designs created significant pressure drop and energy loss. Moreover, the cage and strut restraints projecting from the annular valve body can interfere with heart tissue and make implantation difficult or impossible in certain valve replacement locations. In addition, such restraint structures are difficult to manufacture with the annular valve body in a manner that assures adequate mechanical reliability over years of implantation. Fractures have been reported to have occurred at junctions that were welded together.
A wide variety of pivoting disk heart valve prostheses have been developed and clinically used wherein a single circular disk of pyrolytic carbon cooperates with strut and stop structures to pivot between a disk open position and a disk closed position. The Medtronic Hall.TM. mechanical heart valve employs a strut machined from the titanium block forming the annular heart valve body that is extended through a central opening in the disk to restrain its pivotal movement. Such a single pivoting disk mechanical valve design is reliable, but the opening angle of the disk in the disk open position is limited to less than 90.degree..
More recently, clinically used, bi-leaflet heart valve prostheses have been developed that employ a pair of semi-circular or semi-elliptical plates or leaflets that are coupled to the annular heart valve base or body through pivot hinge mechanisms that allow the leaflets to pivot on leaflet pivot axes between leaflet open and seated, closed positions. The valve body has an interior side wall defining a blood flow orifice having a central blood flow axis centrally located with respect to the interior surface. The valve body also has first and second pairs of valve body hinge elements, e.g. recesses, and first and second valve body seat regions. The pairs of valve body hinge elements provide opposed pairs of hinge pivot points and a pivot axis that extends across the valve annulus and is offset from the central axis of the valve annulus.
In such bi-leaflet valve configurations, two mirror image leaflets are typically disposed in opposed or mirror image relation to one another for alternately blocking blood flow in an inflow direction when seated in a leaflet closed position and then allowing the flow of blood through said blood flow orifice in an outflow direction when in a leaflet open position. Upon closure, each valve leaflet occludes or closes a half section of the annular valve orifice or valve annulus. Generally, each leaflet is generally semi-circular in shape and has generally opposed, inflow and outflow, leaflet major surfaces and a peripheral edge extending between the opposed leaflet major surfaces. A leaflet seat section of the peripheral edge is formed to seat against a valve body seat region when in the closed position. Each leaflet can rotate about a leaflet pivot axis extending between a pair of leaflet hinge elements, e.g., outwardly projecting leaflet ears, that cooperate with a pair of valve body hinge elements, e.g., the opposed pair of hinge recesses. The leaflets are typically planar in profile, but curved or elliptical leaflets have been proposed.
Such mechanical heart valves are typically designed in somewhat differing profile configurations for replacement of different impaired natural heart valves. However, the basic in vivo operating principle is similar regardless of configuration. Using an aortic valve as an example, when blood pressure rises in response to left ventricle contraction or systole in each cardiac cycle, the leaflets of such a valve pivot from a closed position to an open position to permit blood flow past the leaflets in an outflow direction. When the left ventricle contraction is complete, blood tends to flow in the opposite, inflow direction in diastole in response to the back pressure. The back pressure causes the aortic valve leaflets to close in order to maintain arterial pressure in the arterial system.
The most widely accepted type of bi-leaflet heart valve presently used mounts its leaflets for pivoting movement by means of a pair of rounded ears extending radially outwardly from opposed edges of the leaflets to fit within rounded hinge recesses in opposed flat surfaces of the valve body side wall. Such bi-leaflet valves are exemplified by the mitral valve depicted in U.S. Pat. No. 4,276,658 and the aortic heart valve depicted in U.S. Pat. No. 5,178,632, both incorporated herein by reference.
More particularly, the conventional leaflet ears are received within curved hinge recesses extending radially into opposed flat surfaces of thickened wall sections inside the annulus of the generally cylindrical or annular valve body. Each hinge recess is designed in at least one respect to match the shape of the leaflet ear and is bounded by sets of leaflet stop surfaces angled to define the extreme open and closed leaflet positions. In other words, where the ear is formed as a portion of a circle having a given radius, the counterpart hinge recess is formed as a semicircle having a slightly greater radius. An inverse arrangement of the ear and recess hinge mechanism is depicted in U.S. Pat. No. 5,354,330, incorporated herein by reference, whereby the leaflet ear is replaced by a leaflet recess, and the hinge recess is replaced by a complementary shaped hinge boss.
To achieve the pivoting mechanism, the mating surfaces of the ears and recesses are precisely machined so as to provide a small but definite working clearance for the ears to pivot about the necked down pivot surface and be retained within the hinge recesses. During valve assembly, the annular valve body is deformed or distended so that the leaflet ears may be inserted into the respective hinge recesses. Each manufactured heart valve is then lab tested "dry" to ensure that the leaflets are held tightly enough to be secure against falling out of their hinge recesses, but are not so tightly engaged so as to create a binding or restricted valve action.
The range of leaflet motion is typically controlled by pins or ramps or opposed side stops of the hinge recesses or by hinge bosses in the valve body. In one format described in the above-incorporated '632 patent, the hinge recess is generally spherical and bounded by open and closed stop surfaces of a stop member projecting into the recess. In the other formats depicted in the above-incorporated, '658 patent, each hinge recess has an elongated "bow-tie" or "butterfly" appearance created by the inward angulation of opposed side edges extending from inflow and outflow end edges and meeting at opposite disposed, necked down, pivot points or surfaces intermediate the end edges.
A great deal of effort has been devoted to controlling the range of movement and the acceleration of the leaflets between the open and closed positions to both control noise and decrease wear or the possibility of leaflet fracture. Bi-leaflet mechanical heart valves are known to be noisy, in the sense that patients can frequently hear the seating of the valve leaflet peripheral edges against the valve seats upon closure. It is desirable for patient comfort to provide a bi-leaflet design that minimizes the distraction of leaflet seating noise.
It is also known that blood cells are extremely fragile and delicate and can be damaged and/or destroyed when trapped in the valve seat regions during closure of the valve leaflet or in the wiping area of the valve leaflet ears and hinge recesses or between the leaflet ears and the open and closed stop surfaces. The wiping areas of the hinge recesses have the highest potential of thrombus formation and emboli entrapment which can accumulate therein, impair the movement of the valve leaflets, and result in valve failure requiring surgical intervention. To this time, no design has been successful in eradicating this problem. Consequently, patients receiving current bi-leaflet mechanical heart valves are prescribed continuous blood anticoagulation therapy to prevent thrombus formation and thromboemboli. In our commonly assigned U.S. patent application Ser. No. 08/898,144 filed Jul. 22, 1997, and entitled MECHANICAL HEART VALVE PROSTHESIS, we present an improved hinge design that is intended to optimize washing of the hinge regions and decrease these problems of conventional hinge mechanisms of the type described above.
In operation, the valve leaflets accelerate rapidly as the leaflets move from the leaflet open position to the leaflet closed position during the closing phase in response to a change of blood pressure. It is difficult to decelerate the leaflets before the arcuate seat section of the leaflet peripheral edge strikes the corresponding arcuate seat region of the annular valve body. Since a conventional mechanical heart valve leaflet (disk) utilized rigid material, e.g., pyrolytic carbon, the momentum of the rotating rigid leaflet (disk) and its surrounding fluid creates a high impact force due to the sudden stop when the arcuate seat section of the leaflet peripheral edge contacts the corresponding arcuate seat region of the annular valve body. This high impact force damages all blood elements entrapped in the contact region of the leaflet peripheral edge because the impact force is far beyond the bearable limit of any blood element and the dimension of this contact region is two orders of magnitude larger than any blood element. Blood hemolysis in clinical observation is one of the typical results from this high impact force.
Moreover, the blood flow pressure at the inflow side of the conventional mechanical heart valve leaflet peripheral edge can drop to near vacuum pressure due to a water hammer effect upon leaflet closure. At the instant of a leaflet closure, blood volume proximal to the leaflet peripheral edge at the inflow side tends to separate from the leaflet surface due to the moving momentum of fluid column and the abruptly stopping of the rigid leaflet. This flow separation can create a very low pressure in a very short time span, usually less than one milli-second. This very low pressure in the water hammer effect has the potential to generate cavitation which, from occurring to vanished, is less than 50 micron seconds. Material corrosion, pitting and degradation of a leaflet surface caused by cavitation has been observed in clinical use in a few mechanical heart valves. Should cavitation occur, the explosion force of cavitation bubble in a very short time duration can easily damage blood elements near cavitation sites. Even if no cavitation occurs, the low pressure field at the inflow side of the leaflet peripheral edge can cause damage to blood elements by generating high surface tension on surface membranes of blood elements.
Also, after the valve is closed, localized high speed blood flow leakage has been observed on all the current mechanical heart valves. The blood flow leakage jets occur at the gaps between leaflet and valve housing due to the large transvalvular pressure gradient in the valve closing phase. The reported shear stresses of leakage jets are beyond the surface tensile stress limits of any blood element surface membranes. Therefore, these leakage jets not only reduce the efficiency of a passive mechanical heart valve, but also damage blood elements in the leakage stream.